a) 12 1/2 : 1/5 *2,5 = (25/2 *1/5) *2,5 = (5/2*1 *2,5) *2,5 = 2,5*2,5 = 6,25
3 3/4 + 0,28 * 1 11/14 = 3 3/4 + 7/25 * 25/14 = 3 3/4 + 1/2 = 3 3/4 + 2/4 = 3 5/4 = 4 1/4
b) 5,7 : (5 3/7 - 3,8) = 5,7 : ( 5 3/7 - 3 4/5) = 5,7 : ( 5 15/35 - 3 28/35)
= 5,7 : ( 6 50/35 - 3 28/35) = 5,7: 3 22/35 = 57/10 : 127/35 = 57/10 * 35/127 = 57/2 * 7/127 = 399/254 = 1 145/254
